This invention relates generally to meltblowing and in particular to meltblowing dies. In a specific aspect, the invention relates to a die assembly featuring a novel die tip assembly.
Meltblowing is a process in which high velocity hot air (normally referred to as "primary air") is used to blow molten fibers extruded from a die onto a collector to form a web or onto a substrate to form a coating or composite. The process employs a die provided with (a) a plurality of orifices formed in a tip of a triangular shaped die tip and (b) flanking air passages. As extruded strands of the polymer melt emerge from the orifices, the converging high velocity hot air from the air passages stretches and draws them down by drag forces forming microsized filaments.
The filaments are drawn down to their final diameter of 0.5 to 20 microns (avg.) in the case of polyolefin polymers such as polypropylene and to 10 to 200 microns in the case of polymers used in adhesives and spray coating. The strands extruded from the die may be continuous or discontinuous fibers. For the purpose of the present description, the term "filament" refers to both the continuous and discontinuous strands.
The meltblowing process grew out of laboratory research by the Naval Research Laboratory which was published in Naval Research Laboratory Report 4364, "Manufacture of Superfine Organic Fibers", Apr. 15, 1954. Exxon Chemical developed a variety of commercial meltblowing dies, processes, and end-use products as evidenced by U.S. Pat. Nos. 3,650,866; 3,704,198; 3,755,527; 3,825,379; 3,849,241; 3,947,537; and 3,978,185 to name but a few. Other die designs were developed by Beloit and Kimberly Clark. Representative meltblowing patents of these two companies include 3,942,723, 4,100,324, and 4,526,733. Recent meltblowing die improvements are disclosed in U.S. Pat. Nos. 4,889,476, 4,818,463 and 4,889,476.
A key component in the meltblowing die assembly is the die tip which is a machined steel member having a triangular nosepiece through which the orifices are formed. In the die assembly, air passages are formed on opposite sides of the converging triangular nose piece, meeting at the apex where the polymer melt emerges from the orifices. Most of the melt blowing prior art dies employ a long die tip (typically from 10 to 120 inches and longer) having evenly-spaced, side-by-side orifices. In order to provide the desired air drag forces by the primary air on the filaments, the included angle of the nosepiece (which determines the direction of the air flow has been about 60.degree. so that the primary air has a major velocity component parallel to filament spinning.
Also, the meltblowing die assemblies are operated continuously. Interrupting polymer flow presents two problems: (a) polymer continues to dribble out of the polymer orifices, and (b) the air tends to aspirate polymer from the die tips causing undesired afterflow. At the present, when a meltblowing die is shut down, it continues to flow out polymer until the residual polymer in the distribution manifold, the screen pack section and the die tip has emptied itself due to gravitational and aspirational forces. This can be as much as 5 lbs. of melt for conventional dies.
Another feature common to most, if not all, meltblowing dies is the air heating system. Energy used to heat the air is one of the most expensive operational items of meltblowing systems. Generally, the air is compressed and flowed through a furnace and conducted through large insulated conduits to air distribution manifolds on the die assembly. The use of a single furnace for the system not only presents problems in design (because large space must be provided to house the furnace and large conduits) but it also is energy inefficient (because of thermal loss between the furnace and the die assembly). Even small improvements in thermal efficiency can produce large savings in energy costs.
Summarizing the state of prior meltblowing dies, there is a need (a) to provide intermittent polymer discharge from the dies, and (b) to improve the air heater facilities.